1. Field of the Invention
The present invention relates generally to a flip-chip bonding method and an apparatus for flip-chip bonding employing the same, and more particularly, to a method and apparatus for flip-chip bonding semiconductor chips of various sizes by adjusting the diameter and irradiation direction of a laser beam.
2. Description of the Related Art
As electronic products are manufactured with smaller dimensions and increased functionality, integration density and performance of semiconductor chips have increased. To protect a highly integrated, high performance semiconductor chip against various hazardous external environment such as dust, moisture, and electrical or mechanical load, the demand for a lightweight, slim, compact, multi-pin semiconductor package increases.
Because it is difficult to provide compact, multi-pin semiconductor packages using conventional wire bonding, various new bonding techniques have been proposed to address such a limitation. Bump bonding, also called “flip-chip bonding”, is one of the new bonding techniques which includes the steps of: placing bumps (for example solder bumps or gold bumps) on a pad that is an input/output terminal of a semiconductor chip; flipping over the semiconductor chip with the bumps facing down; and attaching the semiconductor chip directly to a carrier substrate or circuit pattern on a circuit tape.
Conventional flip-chip bonding techniques are mainly classified into two categories: thermo compression bonding and laser bonding. As disclosed in Japanese Laid-out Patent Publication No. 2002-141376, thermo compression bonding requires heating of a semiconductor chip for a long period time due to high heat loss in a heat transfer portion, which results in degradation of yield. Because a long time is required to reach the bonding temperature of bumps, this method cannot be applied to a material that is sensitive to high temperature. Another drawback of the thermo compression bonding is that a difference in the thermal expansion coefficient between a semiconductor chip and a substrate may result in a misalignment at a bonded portion between the semiconductor chip and the substrate. Yet another drawback is that damage such as a crack may occur in the bonded portion due to the contraction of the semiconductor chip and substrate after cooling.
Laser bonding involves moving bumps on a semiconductor chip to a bonded position to face designated bumps on a substrate and applying heat using a laser while pressing the semiconductor chip. The use of laser can result in high productivity and low thermal expansion coefficient.
FIG. 1 is a block diagram of a conventional bonding apparatus 10 for bonding lead terminals to bumps of a semiconductor chip by using a laser beam, disclosed in U.S. Pat. No. 5,500,502. Referring to FIG. 1, a semiconductor chip 1A having bumps 2A rests on a supporting base 1 and a lead frame having a plurality of lead terminals 9 is disposed on the semiconductor chip 1A. A laser beam 11 being emitted from a laser irradiation means 3 passes through a shutter 25, is reflected by a dichroic mirror 5, focused by a condenser lens 7 to irradiate and heat the lead terminal 9 and the semiconductor chip 1A such that the lead terminal 9 and the semiconductor chip 1A are bonded together.
A temperature sensor 13 receives infrared rays emitted by the lead terminal 9 while the lead terminal 9 and the semiconductor chip 1A are heated to generate an electric signal. The electric signal generated by the temperature signal 13 is first amplified by an amplifier 15 and then supplied to a filter circuit 17 whereby high frequency components are eliminated. The output signal of the filter circuit 17 is then converted to a digital signal by an analog-to-digital (A/D) converter 19 and inputted to a computer 21. The computer 21 determines the state of a contact between the lead terminal 9 and the bump 2A based on the signal supplied from the A/D converter 19, the result of the determination being displayed on a display unit 23. The computer also determines the state of the bond formed therebetween and the shutter 25 is opened or closed according to the result of the determination.
The conventional bonding using laser allows bonding only within an area on which the laser beam is focused by the condenser lens 7. Another drawback is that a laser beam has a Gaussian intensity profile when irradiated on a surface. Thus, when a laser beam is irradiated at the central portion during bonding, the light intensity is insufficient at the peripheral portion. When the intensity of the laser beam is increased to sufficiently irradiate the peripheral portion, the light intensity becomes extremely high at the central portion. In this case, the energy of a laser beam irradiated on the restricted area through the condenser lens 7 has a non-uniform distribution at central and peripheral portions.
The Gaussian intensity profile makes it difficult to uniformly irradiate the semiconductor chip 9. All bumps 2A on the semiconductor chip 9 cannot be evenly heated, resulting in a non-uniform energy distribution across the semiconductor chip 9. This non-uniform energy distribution leads to distortion due to thermal deformation of the semiconductor chip 9, reduction in bonding strength due to insufficient heating of some bumps 2A, and an increase in the heating time. Thus, there is a need to solve these problems. In particular, these problems become more severe when the sizes of semiconductor chips being mounted on a substrate vary.
In one attempt to solve the foregoing drawbacks and perform bonding over the entire area of the semiconductor chip 1A, a supporting base 1 has to be moved precisely to irradiate with the laser beam the positions where the bumps 2A are located. However, the distance that the supporting base 1 must move to irradiate all of the positions with a focused laser beam increases with the size of the semiconductor chip 1A. Thus, as can be appreciated, when the semiconductor chip 1A has a large size, it is difficult to precisely move the supporting base 1 and, furthermore, the supporting base 1 becomes bulky.
In view of the foregoing, an improved method and apparatus for flip-chip bonding using a laser would be welcomed.